Proton exchange membrane fuel cells are considered to be quite suitable for applications in which the fuel cell power plant will provide electricity to the electric motor of a vehicle. In some applications, such as city buses and package delivery trucks, the vehicle makes frequent stops, mostly for short periods of time. Whenever the electric motor is demanding little or no power from the fuel cell, the current density is very low and the voltage of the cells rise close to open circuit potentials. Although charging of batteries can assist in avoiding the increased potentials, that cannot happen when the state of charge is high.
A practice of shutting down the fuel cell, rather than letting it idle, has seldom been used because once the fuel cell is shut down, it takes a few minutes to get it into full operation again. Thus, the continued preference among operators is for idling, even though oxidation of the catalyst, particularly at the cathode, will occur.